The OFDM modulation technology, with characteristics of high frequency spectrum utilization rate and anti multi-path, has been regarded as a critical technology in the future mobile communication system. The multi-stage modulation scheme using non-continuous amplitudes makes the OFDM system to have a high speed and a high frequency spectrum utilization rate, and at the same time, it also makes the OFDM system to require estimating and tracking the attenuating channel parameters to complete related demodulation.
The channel estimation is used to perform an interpolation function. At present, there are many interpolation methods of channel transformation, such as linear interpolation, second order interpolation, transformation domain interpolation, time domain interpolation, and the like; for details, see “Channel Estimation of OFDM System Based on Comb-like Pilot Frequency Arrangement in Frequency Selective Attenuating Channel” [2] by M. Hsieh and C. Wei on User Electronics, Electric and Electronic Engineer Association Conference, vol. 44, First, February 1998, and “Channel Cstimation Technology Based on Pilot Frequency Arrangement in OFDM System” by Sinem Coleri and Mustafa Ergen on Broadcast, p. 223-229, vol. 48, Third, September 2002. The transformation domain interpolation method and the time domain interpolation technology are briefly introduced hereinafter.
The transformation domain interpolation method comprises the following steps: (1) transforming a channel estimation to a transformation domain, based on pilot frequency symbols calculate a pilot frequency channel estimation by use of a method, such as least square method or linear minimum mean square error method, and then perform a digital Fourier transformation (DFT) operation; and then transform the transformed data sequence into frequency domain by inserting zeros and then perform IDFT (inverse digital Fourier transformation) to complete interpolation operation at the same time.
The channel estimation method [2] [3] using the time domain interpolation technology is a traditional channel estimation method based on the DFT interpolation: after a channel estimation of a pilot frequency sub-carrier is obtained by the LS method or the LMMSE method, transform the channel estimation into time domain to obtain a channel impulse response (CIR) by the IDFT operation, add zeros to the tail part of CIR, and then transform it back into frequency domain by the DFT operation.
The methods for transforming domains are not suitable for long delay multi-path channels, such as UMTS on board type B or COST207 channel, as shown in FIGS. 1(a)-FIGS. 1(c). Because zeros are added in the middle of the DFT transformed sequence, the useful portion of the DFT is divided into two parts, and at the same time, the association between them is broken thus causing estimation error. FIGS. 1(a)-FIGS. 1(c) show the simulation in the case of COST207.
Although the method of performing the IDFT and then adding zeros by using the time domain interpolation technology will not produce errors in the similar transformation process, such method fails to effectively eliminate noise and interference. As shown in FIGS. 2(a)-FIGS. 2(c), the performance of the channel estimation has not been improved.